Use pattern for validating extension names, and correct url to spec.

core/src/main/java/io/cloudevents/core/impl/BaseCloudEventBuilder.java

@@ -30,13 +30,15 @@
 import java.time.OffsetDateTime;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.regex.Pattern;
 
 import static io.cloudevents.core.v03.CloudEventV03.SPECVERSION;
 
 public abstract class BaseCloudEventBuilder<SELF extends BaseCloudEventBuilder<SELF, T>, T extends CloudEvent> implements CloudEventBuilder {
 
     // This is a little trick for enabling fluency
     private final SELF self;
+    private static final Pattern EXTENSION_NAME_PATTERN = Pattern.compile("^([\\da-z])+$");
 
     protected CloudEventData data;
     protected Map<String, Object> extensions = new HashMap<>();
@@ -213,19 +215,10 @@ protected static IllegalStateException createMissingAttributeException(String at
      *
      * @param name the extension name
      * @return true if extension name is valid, false otherwise
-     * @see <a href="https://github.com/cloudevents/spec/blob/master/spec.md#attribute-naming-convention">attribute-naming-convention</a>
+     * @see <a href="https://github.com/cloudevents/spec/blob/main/cloudevents/spec.md#naming-conventions">attribute-naming-conventions</a>
      */
     private static boolean isValidExtensionName(String name) {
-        for (int i = 0; i < name.length(); i++) {
-            if (!isValidChar(name.charAt(i))) {
-                return false;
-            }
-        }
-        return true;
-    }
-
-    private static boolean isValidChar(char c) {
-        return (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9');
+        return EXTENSION_NAME_PATTERN.matcher(name).matches();
     }
 
     protected void requireValidAttributeWrite(String name) {